Optical communications systems typically include a variety of devices (e.g., light sources, photodetectors, switches, optical fibers, amplifiers, filters, and so forth). Optical communications systems are useful for transmitting optical signals over long distances at high speeds. An optical signal, which comprises a series of light pulses, is transmitted from a light source such as a laser to an optical fiber and ultimately to a detector. Amplifiers and filters may be used to propagate the light pulses along the length of the fiber from the light source to the detector. Today, the bulk of long-distance communication traffic is carried by optical fibers. As use of optical fibers becomes more widespread and infiltrates the consumer marketplace, demand is increasing for efficient, high-speed, integrated opto-electronic devices.
There are many considerations and design constraints in developing optical systems. One consideration relates to signal synchronization in a wavelength division multiplexer or demultiplexer and/or optical time-division multiplexer (OTDM) device. Optical communications systems may include such devices for coupling, splitting, or filtering co-propagating pump signals. For example, FIG. 1 reflects a schematic representation of an OTDM system, comprising a time-division multiplexer (TDM) 100 which includes a plurality of low-speed transmitters 102, 103, 104 and a multiplexer 105, an optical switch or demultiplexer 110, and a receiver 115, connected by trunk fiber 11. Each of the transmitters sends low speed signals (sL) to the multiplexer 105 which then outputs a high speed signal (sH) to the switch 110. The switch selectively drops pulses from the high-speed signal to produce low-speed output signals (sO) sent to receiver 115. In this way, the signals may be sent at high-speed over the length of the fiber between the multiplexer 105 and the switch 110, and then interpreted at low speed to determine the information sent from each one of the transmitters.
Such OTDM communication systems require synchronization elements. For example, when the optical switch 110 demultiplexes a high bit-rate signal, a low bit-rate control “C” needs to be synchronized with the high bit-rate signal so that the signals coincide correctly in time inside the switch. This operation requires a delay line, e.g., the control will need to be delayed so it is synchronized inside the switch with the high bit-rate signal. A challenge in designing optical switches involves achieving a delay for a pulse train so that each of the frequencies of the pulse train is delayed for the same period of time. For example, FIG. 2 is a graphical illustration showing the spectrum of an unmodulated pulse train. In FIG. 2, an optical pulse 10 typically comprises a packet of waves 15a, 15b, 15c . . . 15g. Each wave has a certain amplitude and frequency within the bandwidth Δf, e.g., each wave within the packet is characterized by a different frequency and amplitude and travels at a different speed. Challenges are involved in achieving a constant time delay for each of the frequencies over the entire bandwidth Δf. If certain frequencies of the pulse train (e.g. 15a, 15b), are not delayed or are given a different period of delay than other frequencies (e.g., 15c, 15d), the delayed signal will not correspond in phase with the original pulse train.
All-pass filters have been known in the field of electronics for equalizing phase and reducing distortion. Structures for fabricating all-pass filters for electronic devices are known in the field and described in the literature. See, e.g., U.S. Pat. No. 5,258,716 to Kondo et al., “All-Pass Filter.” All-pass filters provide advantages over other types of filters as they affect only the phase of a signal, rather than its amplitude. A configuration for an all-pass filter for use with optical devices is described in co-pending U.S. patent application Ser. No. 09/182,980, titled “All-Pass Optical Filter,” filed by Kazarinov et al. and the inventors herein, which is assigned to the present assignee and incorporated herein by reference. As explained in the Kazarinov et al. application (and shown in FIG. 2B thereof), an optical signal transmitted through a fiber may be distorted or broadened with time over the length of the fiber. This broadening is undesirable as it may create noise, i.e., interference between sequential optical pulses. The Kazarinov et al. application describes an all-pass optical filter designed to eliminate such distortion. Additionally, it was disclosed therein that the all-pass optical filter could be useful in delaying an optical pulse in time. The all-pass optical filter of the Kazarinov et al. application applies a frequency-dependent time delay to each frequency of the optical pulse.
The Kazarinov et al. application describes single-stage and multiple-stage all-pass optical filters. A schematic representation of one embodiment a single-stage all-pass optical filter according to the Kazarinov et al. application is illustrated in FIG. 3. The filter comprises an input port for an input optical pulse 120, an output port 150, a splitter/combiner 143, and a feedback path 145 wherein the feedback path advantageously comprises at least one ring resonator. Although FIG. 3 shows a single-stage filter (e.g., a single resonator ring), the Kazarinov et al. application discloses that best results are achieved when multiple stages (multiple resonator rings) are used. Indeed, the Kazarinov et al. application teaches that many all-pass stages are needed to generate a large tunable delay for an arbitrary broadband signal. For example, FIG. 4 is a graph of the group delay in units of time as a function of frequency for a four-stage all pass optical filter as applied to an arbitrary broadband signal. As can be seen, a maximum and fairly constant delay of 16 au (arbitrary units) is achieved over the normalized frequency range of 0.4 to 0.6. Thus, only certain frequencies would receive the maximum delay. A single all-pass optical filter would achieve a constant delay over a much smaller frequency range (˜0.05) and thus would be ineffective in delaying a pulse train having a large bandwidth (Δf). On the other hand, many all-pass stages would increase the bandwidth (Δf) of the maximum delay period and also lessen the ripple effect. As can be seen, with the four-stage all pass optical filter, a ripple effect is created over the delay period in that four separate summits appear at the maximum height of the delay peak.
However, use of many all-pass stages translates to more complicated systems than if a single-stage all pass filter were used. Preferably, two heaters are deposited on each resonator ring of the device for locally changing the free-spectral range of the group delay and the desired phase. Thus, the four-stage all-pass optical filter used to produce the delay peak shown in FIG. 4 would include the use of eight heaters, each of which would be need to be periodically adjusted depending on the optical signal and desired phase response.
As may be appreciated, those in the field of communications systems continue to seek new designs to improve system performance and reduce cost. In particular, it would be advantageous to have an article comprising a single-stage all-pass optical filter for correcting dispersion and introducing a constant delay. It would be particularly beneficial to provide an all-pass optical filter that can generate a large tunable delay for a broadband signal without use of many all-pass stages.